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A lack of witnesses who saw the entire incident made the case hard to prove

Attorney: Trials are not always about the truth; "It's about what you can prove in court"

Five key issues factored into George Zimmerman's acquittal Saturday in last year's shooting death of 17-year-old Trayvon Martin:

1.The charges filed

Did the prosecution make a mistake in filing a second-degree murder charge against Zimmerman?

"I think the problem was they overcharged it in the very beginning," said Holly Hughes, a criminal defense lawyer who was not on Zimmerman's legal team. Had prosecutors started with the manslaughter charge, the outcome might have been different, she told CNN.

But Florida State Attorney Angela Corey said the allegations "fit the bill" for the second-degree murder charge.

Second-degree murder is defined as a killing carried out with hatred, ill will or spite, but is not premeditated. To convict Zimmerman of manslaughter, jurors would have had to believe he "intentionally committed an act or acts that caused the death of Trayvon Martin."

Photos:Photos: Zimmerman trial

Photos:Photos: Zimmerman trial

Key moments in the Zimmerman trial – George Zimmerman is congratulated by members of his defense team, Don West and Lorna Truett, after the not guilty verdict is read on Saturday, July 13, in Sanford, Florida. A jury of six women found him not guilty in the shooting death of Trayvon Martin. View photos of the public reaction to the verdict.

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Key moments in the Zimmerman trial – George Zimmerman's wife, Shellie Zimmerman, cries as friends and family members celebrate the verdict on July 13.

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Key moments in the Zimmerman trial – Robert Zimmerman Sr. and Gladys Zimmerman embrace after their son is found not guilty on July 13.

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Key moments in the Zimmerman trial – George Zimmerman prepares to leave the courtroom after the not guilty verdict is read on July 13.

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Key moments in the Zimmerman trial – Zimmerman confers with his defense team on July 13, after working out the wording for a response to the jury, who had asked for clarification on the instructions regarding manslaughter. The response, crafted and agreed to by both the prosecution and defense, instructed the jury to ask their question more specifically, as the court could not engage in general discussion.

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Key moments in the Zimmerman trial – Zimmerman and his defense team stand in the courtroom as the jury arrives before starting their second day of deliberations on July 13.

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Key moments in the Zimmerman trial – Prosecutor John Guy addresses the jury with his closing rebuttal during Zimmerman's murder trial on Friday, July 12. "He shot him because he wanted to," Guy told jurors, saying that Zimmerman didn't have to shoot 17-year-old Martin.

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Key moments in the Zimmerman trial – Zimmerman's attorney Mark O'Mara holds up a chart during closing arguments for the defense on Friday, July 12. "How many 'what ifs' have you heard from the state in this case?" O'Mara asked the jury. "They don't get to ask you that."

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Key moments in the Zimmerman trial – Zimmerman, right, sits with another defense attorney, Don West, this week. West objected to a third-degree murder charge also sought by prosecutors on Thursday, July 11, the day closing arguments began. The judge ruled out that charge but said the jury could consider convicting the defendant of manslaughter.

Key moments in the Zimmerman trial – Defense attorney Mark O'Mara, right, questions forensics animation expert Daniel Schumaker, center, at the bench of Judge Debra Nelson with Assistant State Attorney Richard Mantei, left, during a July 9 hearing on the admissibility of animation created for the defense. Schumaker showed the judge and Mantei some 3-D animation on his laptop after an overhead projector didn't work.

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Key moments in the Zimmerman trial – John Donnelly, a friend of George Zimmerman's, cries on the witness stand on Monday, July 8, in Sanford, Florida, after listening to screams on the 911 tape entered in evidence.

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Key moments in the Zimmerman trial – Sondra Osterman, a friend of Zimmerman's, listens to the 911 tape while testifying on July 8.

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Key moments in the Zimmerman trial – Mark Osterman, a friend of Zimmerman's, testifies on July 8 and describes the type of gun Zimmerman owned.

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Key moments in the Zimmerman trial – Leanne Benjamin, a friend of Zimmerman's, smiles while identifying him in court on July 8.

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Key moments in the Zimmerman trial – Sybrina Fulton, mother of Trayvon Martin, takes the stand during Zimmerman's trial on Friday, July 5.

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Key moments in the Zimmerman trial – George Zimmerman's mother, Gladys Zimmerman, listens to the 911 tape while taking the stand during his trial in Seminole County circuit court on July 5.

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Key moments in the Zimmerman trial – Martin's brother Jahvaris Fulton testifies at the Zimmerman trial in Seminole County circuit court on July 5.

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Key moments in the Zimmerman trial – Volusia and Seminole County associate medical examiner Shiping Bao testifies on July 5. Bao conducted the final autopsy on Martin and determined the cause of death to be a gunshot wound to the chest.

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Key moments in the Zimmerman trial – Florida Department of Law Enforcement Crime Lab Analyst Anthony Gorgone testifies about DNA findings on Wednesday, July 3, in Sanford, Florida. Here, Gorgone points to a sweatshirt worn by Trayvon Martin on the night Martin was shot. Only one stain on Martin's hooded jacket yielded a partial DNA profile that matched Zimmerman's.

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Key moments in the Zimmerman trial – Gorgone points to a jacket worn by Zimmerman on the night of the shooting. Multiple stains on Zimmerman's jacket tested positive for Zimmerman's DNA. At least two stains from the jacket tested positive for a mixture of DNA that included Martin's DNA.

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Key moments in the Zimmerman trial – Firearms analyst Amy Siewert from the Florida Department of Law Enforcement answers questions from the prosecution while holding Zimmerman's gun on July 3. Siewert examined the gun and said Zimmerman had one bullet ready to fire in the chamber as well as a fully loaded magazine when the shooting occurred.

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Key moments in the Zimmerman trial – Alexis Carter, a military prosecutor, testifies during the trial on July 3. Carter taught a criminal litigation class that Zimmerman completed, and testified that the class included extensive coverage of Florida's self-defense laws.

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Key moments in the Zimmerman trial – Mark Osterman, a U.S. Air Marshal and friend of Zimmerman's who wrote a book about the case, testifies on Tuesday, July 2. He recounted the story of the shooting that Zimmerman told him and testified that when he took Zimmerman home from the police station after the shooting, Zimmerman wasn't acting like himself.

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Key moments in the Zimmerman trial – Prosecutor Bernie de la Rionda, on July 2, demonstrates a possible scenario while questioning state witness Chris Serino, a Sanford police officer.

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Key moments in the Zimmerman trial – Hirotaka Nakasone, a voice recognition expert with the FBI, testifies in the Zimmerman trial on Monday, July 1.

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Key moments in the Zimmerman trial – Witness Jonathan Good is cross-examined by defense attorney Mark O'Mara on Friday, June 28.

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Key moments in the Zimmerman trial – Selma Mora reenacts a scenario for defense attorney Mark O'Mara on Thursday, June 27. Mora lived in Zimmerman's neighborhood at the time of the shooting.

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Key moments in the Zimmerman trial – Witness Jennifer Lauer points to where her former home was in the Retreat at Twin Lakes community during questioning by defense attorney Mark O'Mara on June 27. Lauer called 911 on the night of the shooting.

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Key moments in the Zimmerman trial – Rachel Jeantel, a friend of Martin's, is questioned by defense attorney Don West on June 27. She appeared to get frustrated several times during the cross-examination, including one time when West suggested they could break until the morning so she'd have more time to review the deposition transcript.

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Key moments in the Zimmerman trial – The evidence letter that Jeantel says she wrote with a friend for Sybrina Fulton, Martin's mother, is displayed during the trial on June 27. When the defense asked Jeantel to read the letter, she said she couldn't read cursive. She asked a friend to write the letter for her, she said.

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Key moments in the Zimmerman trial – Jeantel testifies on Wednesday, June 26. She was the last person to speak with Martin on the phone.

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Key moments in the Zimmerman trial – Zimmerman walks past Martin's parents, Sybrina Fulton, left, and Tracy Martin, second from left, as he enters the courtroom after lunch recess on June 26.

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Key moments in the Zimmerman trial – Diana Smith of the Sanford Police Department on Tuesday, June 25, shows the jury a bag of Skittles that was collected as evidence at the crime scene. Martin was said to be carrying the bag of candy and a soft drink at the time of his death.

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Key moments in the Zimmerman trial – Assistant state attorneys John Guy, left, and Richard Mantei hold up Martin's sweatshirt as evidence during Zimmerman's trial on June 25. After Martin's death, protesters started wearing hoodies in solidarity against racial profiling.

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Key moments in the Zimmerman trial – During the trial on June 25, crime scene technician Diana Smith shows the jury a gun that was collected as evidence.

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Key moments in the Zimmerman trial – Zimmerman laughs with defense attorney Don West during his trial on June 25.

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Key moments in the Zimmerman trial – Selene Bahadoor enters the courtroom to take the witness stand on June 25. She was the first eyewitness to testify and said the shooting occured right behind her home.

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Key moments in the Zimmerman trial – Seminole County 911 dispatcher Sean Noffke testifies on Monday, June 24, about his conversation with Zimmerman on a non-emergency line the night of the shooting.

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Key moments in the Zimmerman trial – A transcript of Zimmerman's police call on the night of the shooting is projected during opening arguments on June 24.

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Key moments in the Zimmerman trial – Martin's father, Tracy Martin, cries on June 24 as he listens to the description of his son's death.

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Key moments in the Zimmerman trial – Prosecutor John Guy gestures during his opening arguments on June 24. His first words to the six-woman jury may have raised a few eyebrows. "Good morning. 'F*****g punks, these a******s all get away,'" Guy quoted Zimmerman. "These were the words in this grown man's mouth as he followed this boy that he didn't know. Those were his words, not mine."

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Key moments in the Zimmerman trial – From left, Zimmerman's father, Robert Zimmerman Sr.; his mother, Gladys; and his wife, Shellie, are escorted from the courtroom on June 24. Since they are all on the witness list, the judge ruled they cannot be present in the courtroom until after they testify.

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Key moments in the Zimmerman trial – Defense attorney Don West displays a photo of Zimmerman from the night of the shooting during his opening arguments on June 24. He opened his statements with a knock-knock joke but failed to win a laugh. "Knock knock. Who's there? George Zimmerman. George Zimmerman who? Good, you're on the jury," he said.

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Key moments in the Zimmerman trial – A video entered as evidence is displayed on June 24. It shows Martin, right, at a 7-Eleven on the night of his shooting.

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Key moments in the Zimmerman trial – From left, Martin's parents, Tracy Martin and Sybrina Fulton, and Benjamin Crump, the family's legal counsel, make a brief statement to the media before jurors heard opening statements on June 24.

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Key moments in the Zimmerman trial – Zimmerman waits for the start of his trial on June 24.

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Photos:Photos: Reaction to Zimmerman verdict

Photos:Photos: Reaction to Zimmerman verdict

Reaction to Zimmerman verdict – Sybrina Fulton, mother of Trayvon Martin, is joined by her son Jahvaris Fulton as she speaks to the crowd during a rally in New York City, Saturday, July 20. A jury in Florida acquitted Zimmerman of all charges related to the shooting death of Trayvon Martin. View photos of key moments from the trial.

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Reaction to Zimmerman verdict – Protesters attend a rally in support of Trayvon Martin, in New York on July 20. The Rev. Al Sharpton's National Action Network organized the "'Justice for Trayvon' 100 city vigil" which called supporters to gather in front of federal buildings around the country on July 20, as a continued protest against the George Zimmerman verdict.

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Reaction to Zimmerman verdict – The Rev. Al Sharpton speaks to the crowd during the rally in New York City on July 20.

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Reaction to Zimmerman verdict – Beyoncé, left, and Jay-Z, center, arrive at the rally in New York City on July 20.

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Reaction to Zimmerman verdict – Thousands of people gathered outside the Richard B. Russell Federal Building in downtown Atlanta as part of the network of vigils on July 20.

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Reaction to Zimmerman verdict – A chalk outline, a bag of Skittles, and a can of iced tea are seen during the vigil outside the E. Barrett Prettyman Federal Courthouse in Washington, D.C., on July 20.

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Reaction to Zimmerman verdict – Protesters march across the Brooklyn Bridge toward Brooklyn after attending the rally in Manhattan on July 20.

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Reaction to Zimmerman verdict – Tracy Martin, father of Trayvon Martin, poses for a photo with supporters wearing hoodies at the rally in Miami on July 20.

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Reaction to Zimmerman verdict – Protesters march through the streets of downtown Los Angeles, on Tuesday, July 16, during a demonstration of the George Zimmerman trial.

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Reaction to Zimmerman verdict – The Rev. Al Sharpton calls for a full federal investigation of the Martin killing, saying mere remarks by President Barack Obama and others weren't enough, outside the U.S. Justice Department in Washington on July 16.

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Reaction to Zimmerman verdict – A passenger takes a picture of protesters as he rides a city bus on July 16 in Los Angeles.

Reaction to Zimmerman verdict – Leon McCutchin participates in a candlelight vigil for Martin on July 15 in New York City.

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Reaction to Zimmerman verdict – A large group of demonstrators march through downtown Atlanta on July 15 during a protest of the acquittal of George Zimmerman.

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Reaction to Zimmerman verdict – Outside the Department of Justice in Washington on July 15, Rev. Anthony Evans, president of the National Black Church Initiative, leads a prayer during a demonstration asking for justice for Trayvon Martin.

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Reaction to Zimmerman verdict – Residents of Sanford, Florida, attend a prayer vigil to promote peace and unity in their city in the wake of the George Zimmerman trial on July 15.

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Reaction to Zimmerman verdict – About 500 demonstrators gather during a rally and march in support of Trayvon Martin on July 15 in Birmingham, Alabama.

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Reaction to Zimmerman verdict – A man argues with a police officer as supporters of Trayvon Martin march while blocking traffic in Union Square in New York on Sunday, July 14.

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Reaction to Zimmerman verdict – A man throws a trashcan during a protest in Oakland, California, on July 14.

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Reaction to Zimmerman verdict – People gather at a rally honoring Trayvon Martin at Union Square in New York on July 14.

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Reaction to Zimmerman verdict – Demonstrators march following a rally at the Torch of Freedom in downtown Miami on July 14.

Questions surfaced about whether the charge was appropriate because knowing a person's feeling during a killing is difficult to prove.

As a result, the prosecution had to rely on the evidence to tell the story of what happened and what led to the shooting, HLN's Ryan Smith said.

Without an abundance of evidence, proving the case was difficult for both sides.

Former Los Angeles County prosecutor Loni Coombs emphasized that jurors did not find Zimmerman innocent; rather, they found him not guilty. There's a difference, she told CNN's Don Lemon.

"They're not saying he's innocent, they're just saying they couldn't prove beyond a reasonable doubt, and therefore the law gave them no choice but to write 'not guilty,'" Coombs said.

2.The evidence presented

Zimmerman's account of what happened the night of the shooting was a central part of the trial. He was the only living person who witnessed the entire incident, and there wasn't much physical evidence for either team to fall back on.

Various adverse conditions played into the initial investigation that night: A dark, rainy scene isn't ideal for a homicide investigation, says HLN law enforcement analyst Mike Brooks.

Much was made during the trial about why there was so little blood on Martin and Zimmerman, who said he fired in self defense because he was being beaten and feared for his life.

Dr. Vincent Di Maio, a former medical examiner in San Antonio and an expert on gunshot wounds, testified that the rain could have washed away and affected evidence collected from Martin's hands.

Attorney Faith Jenkins said it doesn't matter what a prosecutor believes if he or she doesn't have adequate evidence.

"Trials are not necessarily about the truth all the time," the former prosecutor told CNN. "It's about what you can prove in court."

There was so little evidence available that the defense put together an animated video to re-enact the events of that night based on witness statements, police reports and Zimmerman's account, they said.

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Jeantel testified that Martin told her he was being chased by a "creepy-a** cracker."

"This was a disaster," criminal defense lawyer Mark Geragos told CNN's Anderson Cooper the day Jeantel testified. "This was the star witness, the star witness. The wheels came off and it was a train wreck. And there's no other -- there's no way to soft-pedal it."

The mothers of Zimmerman and Martin each testified that screams for help heard on a 911 call the night of the shooting were those of her son.

Whose voice was heard on the call was considered key for both the prosecution and defense in proving who was responsible for the shooting. If the screams were Martin's, as the prosecution contended, Zimmerman was the aggressor. If the screams were Zimmerman's, as the defense said, Martin was the aggressor.

On July 5, in a packed courtroom, the prosecution played the recording for Martin's mother, Sybrina Fulton.

When asked if she recognized the voice, Fulton said it belonged to "Trayvon Benjamin Martin."

Hours later, the defense called Gladys Zimmerman to the stand, where she testified the voice screaming for help belonged to her son.

"I know because he's my son," she said.

As a result, there was no definitive answer for jurors.

5.Testimony

The lead detective in the case, Chris Serino, was called to the stand by the prosecution.

Serino told the court that he believed Zimmerman exaggerated the number of times he was hit that night but didn't feel any "active deception."

"Either he was telling the truth or he was a complete pathological liar," he testified.